Injection compression molding engineering is the technique in which the resin is injected into the slightly opened mold followed by a mold clamping operation to accomplish the molding with the following advantages.
Since the cavity may be filled with an injection pressure lower by several fold than the pressure applied ordinarily for the injection molding, it will require less mold clamping pressure.
A sufficient pressure may be applied to the molten material in the cavity so that relatively finer portions of the mold may be filled with a sufficient material to improve the transferability to the mold.
Less residual stress and fluidity orientation may prevent deformation of product to improve accuracy of the shape.
Sufficient molding pressure to the molten material with enhanced effect of swelling may prevent a sink mark of the thicker product. Such engineering is known to be preferred to manufacture the products which require greater thickness and high accuracy in shape, particularly optical parts such as plastic lens and disk, and various kinds of thicker molded products.
In the actual compression molding operation with the specific resin material and the mold of the specific shape, individual mold clamping pressure control conditions vary depending upon the characteristics of resin, shape of the mold, molten resin or the temperature conditions of the equipment and it was impossible to determine generally how much clamping pressure should be applied at what point during the compression molding operation, for which reason in the conventional injection compression molding operation, the setting of the mold clamping pressure control condition corresponding to the individual cases has always relied on experience. In other words, the shrinkage characteristics of the applicable resin, the temperature setting of the mold temperature controller, the cooling effect, the temperature variation characteristics of the cavity section, variation characteristics of the mechanical motion accompanied by the temperature change of the hydraulic fluid and the like were empirically judged by skill to determine the status of variation in the mold clamping pressure during the compression molding operation. For example, by the timer setting a program in which the mold clamping pressure is varied with time is provided.
According to the conventional injection compression molding operation which sets the compression molding condition for individual cases as a program, it was in fact impossible to cope with the variation in external environment including variation in operational environment particularly, variation in an outdoor temperature, drying extent of the resin and the like in addition to the problems involved in the program per se. When the mold clamping pressure control takes place under the preset program which progresses separately irrespective of the variation in phase of the resin having been filled in the cavity and being in transition from the molten condition to solidification, nonconformity occurs between the variation in phase of the resin and the program due to various factors, so that the resin is occasionally subjected not to fixed pressure intended to be applied under a certain condition but rather to the pressure appropriate for another condition. Accordingly, when the pressure rises during solidifying of the resin from the molten condition, the residual stress generates in the molded product a problem of strength and also problems of internal strain in the optical parts resulting in variation of the light refractive index.
The conventional mold clamping pressure control for the injection compression molding operation requires a certain skill for setting of the control condition per se and gives rise to problem that may not be coped with in individual cases such as for the program which progresses by variation in an external environment and the like even when the necessary conditions have been provided. The aforementioned advantages of the conventional molding technique expected for the injection compression molding operation have not met with effective utilization but with generation of defective molded product as the case may be, for which reason realization of incorporating the characteristics of resin per se fully into the mold clamping pressure control which is not affected by operational conditions, external environments and the like has been intensively desired.